Amphoteric liposomes have been found to exhibit excellent biodistribution and to be well tolerated in animals. They can encapsulate active agents, including nucleic acid molecules, with high efficiency.
In contrast to zwitterionic structures, amphoteric liposomes advantageously have an isoelectric point and are negatively charged at higher pH values and positively charged at lower pH values. Amphoteric liposomes belong to the larger group of pH-sensitive liposomes that were introduced by Straubinger, et al. (FEES Lett., 1985, 179(1), 148-154). Typical pH-responsive elements in pH-sensitive liposomes are cholesterol hemisuccinate (CHEMS), palmitoylhomocysteine, dioleoylglycerol hemisuccinate (DOG-Succ) and the like. CHEMS can stabilise dioleoylphosphatidylethanolamine (DOPE), a lipid which preferentially adopts the inverted hexagonal phase at temperatures above 10° C., into the lamellar phase at pH 7.4. Lamellar CHEMS/DOPE systems can be prepared at neutral or slightly alkaline pH but these systems become unstable and fuse at acidic pH (Hafez and Cullis, Biochim. Biophys. Acta, 2000, 1463, 107-114).
Fusogenic liposomes are very useful in pharmaceutical applications, especially for the intracellular delivery of drugs, e.g., nucleic acids, such, for example, as plasmids and oligonucleotides. After the uptake of a liposome into a cell by endocytosis the release of the drug from the endosome is a crucial step for the delivery of a drug into the cytosol of cells. The pH within an endosome is slightly acidic and therefore pH sensitive liposomes can fuse with the endosomal membrane and thereby allowing the release of the drug from the endosome. This means that destabilisation of the lipid phase, e.g., by enhanced fusogenicity, facilitates endosome escape and intracellular delivery. Also other environments of low pH can trigger the fusion of such liposomes, e.g., the low pH found in tumors or sites of inflammation.
Hafez, et al. (Biophys. J. 2000, 79(3), 1438-1446) were unsatisfied with the limited control over the pH at which such fusion occurs and demonstrated a rational approach to fine-tune the fusion point by adding cationic lipids. Such mixtures have true amphoteric properties in that they exist in a cationic state at low pH and as anionic particles at higher pH, typically at physiological pH. According to Hafez, et al. fusion starts at pH values where the net charge of the particles is zero (their isoelectric point), and once such point is crossed (the pH is lower to any extent) fusion is a continuous process. This view is shared by Li and Schick (Biophys. J., 2001, 80, 1703-1711) who analysed the fusion tendency for amphoteric lipid mixtures using a mathematical model.
Israelachvili and Mitchell in 1975 (Biochim. Biophys. Acta, 1975, 389, 13-19) introduced the molecular shape concept which assumes that the overall form of lipid molecules determines the structure of the hydrated lipid membrane. This means that the lipid geometry and more specifically the size ratio between the polar head-group and the hydrophobic membrane anchor is the key parameter determining the lipid phase (Israelachvili, et al. Biochim Biophys Acta. 1977 17; 470(2):185-201). The original theory however did not consider counterions being a steric part of the polar head-group, but this was contributed by Li and Schick (Biophys. J., 2001, 80, 1703-1711). In their description of the DODAC/CHEMS system, the sodium ion enlarges the head-group of CHEMS at neutral pH, but dissociates as the pH drops, thus minimising the head-group volume and promoting a hexagonal phase; DODAC as a strong cation is assumed to be in constant association with its respective counterion, irrespective of the pH. The model predicts fusion at some pH and below.
Lipid phases according to the molecular shape concept (Israelachvili et al., 1980, Q. Rev. Biophys., 13(2), 121-200):
ShapeOrganisationLipid phaseExamplesInvertedMicellesIsotropicDetergentsconeHexagonal ILysophopholipidsCylinderBilayerLamellarPC, PS, PI, SM(Cubic)ConeReverseHexagonal IIPE, PA at lowmicellespH or with Ca2+,Cholesterol,Cardiolipin
The addition of neutral lipids to amphoteric lipid mixtures has been found to have little impact on the isoelectric point of amphoteric liposomes. WO 02/066012 (Panzner, et al.) discloses certain amphoteric liposomes comprising neutral lipids with a stable size at both low and neutral pHs. WO 02/066012 also describes a method of loading such particles with nucleic acids starting from a low pH.
Amphoteric liposomes are complex structures and comprise at least a complementary pair of charged lipids. The inclusion of one or more such neutral lipids significantly adds to the complexity of the mixture, especially since the individual amounts of the components may vary. Hafez, et al. (Biophys. J. 2000, 79(3), 1438-1446) and WO 02/066012 provide some guidance as to how to select lipid mixtures with truly amphoteric properties and more specifically how to determine their isoelectric point and onset of fusion. Nevertheless, the very high number of possible combinations of lipids represents a practical hurdle towards a more rapid optimisation of amphoteric liposomes, and there remains a need in the art for a method of predicting or analysing which mixtures of lipids form satisfactorily stable lamellar phases at high and low pH, whilst forming a fusogenic, hexagonal phase at an intermediate pH.
It is an object of the present invention therefore to provide improved methods for formulating such fusogenic amphoteric liposomes. Amphoteric liposomes that form a stable lipid phase at neutral pH and a fusogenic phase at low pH represent another object of the invention. Yet another object of the invention is the provision of amphoteric liposomes that form stable lipid phases both at low pH and at neutral pH, but undergo fusion at an intermediate pH. The inventors have recognised that it would be desirable to control the pH at which an amphoteric liposome is fusogenic, so as to enable the liposome to be better targeted in some applications to the endosomal environment where its cargo is desired to be released. Yet another object of the invention therefore is to provide a way of controlling the pH at which fusion of such amphoteric liposomes occurs.